Respiratory Systems
Question #1
- Why is the largest insect smaller than a mouse?
Question #2
- Why do humans drown in water,
but fish don’t?
All gas exchange is by diffusion
- All
organisms exchange gases (O2, CO2)
with their environments by diffusion.
- Diffusion is the net movement of molecules from areas of higher concentration to areas where they are in lower concentration.
- Diffusion of water molecules is "osmosis."
Requirements for gas exchange
Four things are needed for gas exchange:
- A concentration gradient
- A moist membrane
- Adequate surface area
- Internal transport system
Concentration gradient
- Diffusion occurs only down a concentration gradient, from areas of high concentration to areas of low concentration.
- For example, the diffusion of O2 from your lungs into your blood.
Moist membranes
- Diffusion occurs only across moist membranes.
- If aquatic, gas exchange surfaces can be external (e.g., gills).
- If terrestrial, gas exchange surfaces are usually internal (e.g., lungs).
If small, diffusion alone is enough
- Diffusion through body fluids is very slow.
- Diffusion alone is good enough for gas exchange IF the distances O2 has to diffuse
are short (< 1 mm)
- If small (e.g., Paramecium, flatworms),
diffusion alone is enough.
In larger organisms
- Diffusion alone cannot meet the needs of the inner-most cells of larger organisms.
- Larger organisms must have some form of internal transport system for gases.
The tracheal system of insects
- Insects have a system of small, highly branching air tubes that carry gases in and out of their bodies.
Tracheal system of insects
- Insects have a gas transport system made
of tracheal tubes and spiracles.
- In the tracheal tubes, O2 and CO2 diffuse in/out along their concentration gradients.
- Spiracles close to reduce loss of moisture.
No insects larger than mice?
- Diffusion of O2 and CO2 through air in the tracheal tubes is fast enough only for distances < 1 cm from the surface.
- This limits the "radius" of insect’s body.
- Larger organisms use a blood circulatory system to transport gases.
Adequate gas exchange surfaces
- Gas exchange surfaces (lungs, gills) are often highly branched, to increase the organism’s surface-area-to volume ratio.
- SA = area exposed to the environment.
- V = the volume of cells needing oxygen.
- Organisms need an adequate SA/V ratio.
Question #2
- Why do humans drown in water,
- but fish don’t?
Lungs
- Lungs have a highly-branched system of air tubes, from trachea to bronchioles
Lungs
- Highly-branched (trachea-->bronchioles)
- 100,000s of alveoli (air sacs)
- 40 times the surface area of the body
Alveoli
- Air sacs surrounded by capillaries.
- O2 and CO2 diffuse in opposite directions;
both down their concentration gradients.
Respiratory ailments
- Emphysema is the loss of alveolar sac complexity => reduces surface area for
gas exchange.
- Asthma is the constriction of smooth muscle surrounding the bronchioles => reduces volume of air flow in and out.
Gills
- Continuous flow of water over gills
(into mouth-->out under gill covers)
Gills
- Highly branched:
gill arch --> filament --> lamella
Countercurrent flow in the gills
- Water & blood flow in opposite directions.
- If water & blood flowed in the same direction, the gills would extract only
50% of the oxygen dissolved in the water.
- Countercurrent flow can increase potential extraction efficiency to nearly 100%.
If flows are in the same direction.
If flows are in opposite directions
Why don’t fish drown?
- Oxygen is 21% of air, < 1% in water
- More water can flow over (external) gills than in and out of (internal) lungs.
- Countercurrent flow in gills extracts more of the oxygen from the water.
- Metabolic rate of fish is lower.
Circulatory Systems
- Circulatory systems have evolved ways
to increase gas transport efficiency.
Gas transport efficiency of blood
- Only limited amounts of gases can dissolve in blood (like a warm, open Coke)
- Respiratory pigments (e.g., hemoglobin) increase carrying capacity of blood for O2
- Red blood cells = 250,000 hemoglobin molecules per rbc.
Open Circulatory Systems
- Blood bathes the body tissues directly.
- Blood does not remain inside blood vessels for its entire circuit.
- There is no distinction between "blood"
and "interstitial fluid."
Open circulation in insects
Open circulation in insects
- Dorsal heart(s) pump body fluids
- Sluggish flow through body cavities
- Efficient enough for transport of food and wastes. (How are gases transported?)
Closed Circulatory Systems
- In which the blood is confined to blood vessels through the entire circuit.
- There is a distinction between blood and interstitial fluid.
Closed circulation in earthworm
Closed circulation in earthworm
- Dorsal and ventral vessels connected
by 5 pairs of pulsating tubes ("hearts")
- Gas exchange is through capillaries in body wall under moist, mucous-covered skin
- If earthworm is too wet…
- If earthworm is too dry...
Key Points
- Requirements for gas exchange to occur.
- Various adaptations have increased the efficiency of gas exchange surfaces.
- Other adaptations have increased the efficiency of internal gas transport.
Next lecture
- Increasing pumping efficiency of the heart
- Evolution of our 4-chambered heart
- Coordination of contraction (ECGs)
- (High) blood pressure
- Other circulatory ailments